Thrombin and collagen induce rapid phosphorylation of a common set of cellular proteins on tyrosine in human platelets

The ability of thrombin and collagen to induce protein-tyrosine phosphorylation in intact human platelets was assessed by using antibodies to phosphotyrosine in conjugation with immunoblots. Upon stimulation by thrombin there was an increase in the amount of protein-tyrosine phosphorylation of three bands with molecular masses of 135, 124, and 76 kDa in a time-dependent manner. The tyrosine phosphorylation in these three proteins increased in a concurrent fashion and reached a maximum level in 10 s and then a plateau or a slight decrease. Stimulation by collagen was also followed by an increase in tyrosine phosphorylation of 135- and 124-kDa proteins. Unlike stimulation by thrombin, collagen induced no obvious tyrosine phosphorylation of 76-kDa protein. The time courses for thrombin- or collagen-induced protein-tyrosine phosphorylation were similar to that for [14C] serotonin release. These results suggest that 135- and 124-kDa proteins are a common set of proteins that become phosphorylated on tyrosine residue during platelet activation.